Metering circuit for Sine/Square Generator

The Sine/Square Wave Generator described in the February 2000 issue has been quite a popular project but we have had requests for a metering circuit for it. Both this and a frequency readout were left out of the original design to keep the design simple and low in cost. In fact, since the square output has a fixed amplitude, there is no point in monitoring its amplitude since it will always be 5V peak-to-peak, as set by the 5V supply and the CMOS circuitry.

However, it would be worthwhile having a metering circuit for the sine­wave output which is adjustable in level from 0-2V RMS. The accompanying passive circuit, involving two ger­manium diodes and a 100µA meter, can accomplish this.

The 100µA meter movement is connected inside a bridge cir­cuit consisting of two germanium diodes and two 10k resistors. The input impedance of the metering circuit will be quite high, above 20k, and therefore not cause any loading problems on the output of the Sine/Square Wave Generator circuit.

While we have specified OA91s on the circuit, virtually any germanium signal diodes can be used in this application.

Trimpot VR1 is provided for calibration against a digital multi­meter. Calibration should be done at a low frequency (eg, 100Hz), to ensure that the DMM’s bandwidth does not prejudice the meas­urement.

SILICON CHIP.

Crystal timebase for capacitance meter

This crystal oscillator replaces the 7555 timer as the 950kHz timebase in the Digital Capacitance Meter published in the May 1990 issue of SILICON CHIP. The new circuit has a transistor oscillator based on a 4.75MHz crystal and this is fed to a 74HC390 which is set up to divide by a factor of five, giving the wanted frequency of 950kHz. This is fed to pin 1 of IC5 in the original circuit.

To take advantage of the lower drift and greater precision of the new timebase, the Capacitance Meter should be recal­ibrated by trimming the charge resistors selected by switches S2 and S1a. This should be done using a close tolerance (1% or better) standard capaci­tor; eg, 0.1µF.

Gregory Freeman,

Mt Baxter, SA. ($30)